In the fabrication of multi-layer ceramic substrates for use in semiconductor packages of the type described in U.S. Pat. No. 4,245,273, a mixture of ceramic particles, a resin binder, and a solvent for the binder is doctor bladed into thin sheets and then dried. The resultant green ceramic sheets are punched to form via holes, the via holes filled and circuit patterns imprinted with a conductive paste, the sheets assembled into a laminated structure and the resultant substrate sintered to burn away the binder and solvent and fuse the ceramic particles. After the sintering operation, metallurgy patterns are formed on the top and bottom surface to make contact with and support suitable I/O connections. These connections are used to make electrical connections to semiconductor devices, normally on the top surface, and connections to a supporting board, card, or other support on the bottom surface. The internal metal of the substrate must withstand the high temperature sintering operation. This normally requires the use of a refractory metal. However, these metals cannot be conveniently joined to I/O and device elements by solder and brazing techniques. What is commonly done is depositing additional metallurgy patterns of metals that are solderable and compatible with brazing operations over the refractory metal vias and patterns.
However, during the sintering operation, green ceramic substrate shrinks substantially, usually on the order of 15 to 20%. Unfortunately, the shrinkage is not always uniform resulting in a distorted pattern of refractory metal vias and other patterns on the sintered substrate surface. When the geometry of the refractory metal pattern is small, as it must be in high performance semiconductor substrates, and the substrate is of substantial size, the subsequent metallurgy pattern cannot be deposited by conventional evaporation through a mask or blanket evaporation followed by subtractive etching because it requires alignment of a mask with the underlying refractory metallurgy pattern. The distortion occurring during sintering thus precludes mask alignment to the pattern.
In order to apply the necessary non-refractory metal layers over the refractory metal areas on a substrate, the metals must be deposited by electroless and immersion plating or by electroplating techniques. Electroplating has not been generally used because this type of deposition requires establishing electrical contact to the specific areas to be plated in order to make them the cathode. Making this electrical contact is not always possible because some of the pad areas may be electrically floating. Electroless plating is a form of chemical plating which involves reduction of a metal salt to the metal with the simultaneous oxidation of a chemical compound called a reducing agent. To prevent or at least minimize the tendency for the oxidation-reduction reaction to take place throughout the plating solution, electroless plating solutions are formulated so that the concentration of the metal salt and/or reducing agent and Ph are such that the metal reduction does not occur readily. This being so, the areas to be plated would not be plated either. This problem is overcome by the use of catalysts which localize the plating reaction to the desired surfaces only. In the case of the ceramic substrates, the refractory metal surface areas are catalyzed to promote the oxidation-reduction reaction, whereas the ceramic material areas are not. The metal is thus selectively deposited only on and over the refractory metal areas.
During the electroless plating operation, there frequently occurs extraneous metal deposition that does not overlie the refractory metal areas. This extraneous plating is unacceptable since electrical shorting will occur between metal pads that must be electrically isolated. In order to remove these extraneous metal areas of non-refractory metal, the entire metallurgy layer over the refractory metal is removed and the electroless deposition process repeated. This rework is time-consuming and quite expensive since a plurality of non-refractory metals are normally deposited over the refractory metal. Also, it has been found that the number of times that the rework operation can be repeated is low, sometimes only one time, before the entire substrate is degraded to the point that it must be discarded. At this point in time, the multi-layer ceramic substrate is nearly complete and represents a relatively large investment.
What is needed in the packaging industry is a simple and inexpensive process for selectively removing extraneous metal that does not overlie the refractory metal layer area. The rework process must not degrade the substrate or the refractory metal areas and must not involve use of masking for the reasons previously discussed.
The prior art, U.S. Pat. No. 3,698,941, discloses a method of applying contacts to a semiconductor body wherein the semiconductor surface, in which the surface is partly covered by an insulating layer, has deposited a metal layer applied to the entire surface. The metal layer is heated to increase the adherence of the metal layer to the semiconductor material and is subsequently subjected to acoustic high-frequency vibrations to remove the metal layer on portions overlying the insulating layer.